Known in the art is a process for desilication of aluminate solutions involving the use of additives such as compounds of alkali-earth metals including dolomite (cf. FRG Pat. No. 752,739 and Japanese Pat. No. 28297/70 of Nov. 17, 1965 Cl. 15F 251.1).
However, this process has not been commercially realized, since the degree of desilication of an aluminate solution does not exceed 60-80%. Furthermore, the process proceeds at a very slow speed.
In order to intensify the desilication process and increase the degree of desilication of aluminate solutions it has been suggested to calcine dolomite ground to a fineness of 50 mesh in a rotary furnace at a temperature within the range of from 1,500.degree. to 1,600.degree. C. during calcination of dolomite, 0.5-6% of iron powder is added thereto thus resulting in the formation of iron scale at the surface of dolomite particles. The use of such a desilication agent results in a degree of desilication of solutions as high as 99.7%. The desilication process is performed at a temperature within the range of from 100 to 300.degree. C. under a pressure of from 1 to 150 atm, over the heating time about 0.5 hour and more in the presence of 1-50 g/l of dolomite. The best results are obtained at a temperature of from 150.degree. to 200.degree. C., pressure of 1 to 20 atm, process duration of from 1 to 3 hours and addition of dolomite in an amount of 5-10 g/l. It is also possible to perform the process in two stages. (Cf. U.S. Pat. No. 2,442,226 of Mar. 14, 1944).
However, practical use of this process has shown that the degree of desilication was only 90-92%; the solutions were contaminated with iron oxide thus hindering the manufacture of top-grade alumina. Furthermore, the use of elevated temperatures in the process results in considerable power and material consumption rates, i.e. 2 Gcal of steam and 300-400 kg of Al.sub.2 O.sub.3 per ton of dolomite.
Also known in the art is a process for desilication of aluminate solutions involving treatment thereof with lime. The effect of lime in the desilication treatment is based on cation exchange accompanied by the formation of an alumosilicate, wherein sodium ions are substituted with calcium ions; the composition of calcium alumosilicate in white slime is CuO.Al.sub.2 O.sub.3.17SiO.sub.2.nH.sub.2 O.
A lesser solubility thereof in aluminate solutions causes a better desilication in the presence of lime. In doing so, as it has been experimentally shown, losses of Na.sub.2 O in white slime are smaller, though losses of Al.sub.2 O.sub.3 are greater. The reason of the phenomenon resides in that lime is always added in excess for a better elimination of SiO.sub.2, though this also results in the evolution, along with alumosilicates, of calcium hydroaluminate as well. The stronger aluminate solutions, the higher are losses of Al.sub.2 O.sub.3 per unit weight of silica. In the presence of lime, precipitated from concentrated solutions upon desilication are solid solutions consisting of lime hydroaluminate and sodium alumosilicate thus causing increased losses of Al.sub.2 O.sub.4.
At the same time, losses of Na.sub.2 O are also increased approaching the losses observed for desilication without lime. The stronger aluminate solutions, the higher is the amount of NaOH therein and the worse the desilication process.
Therefore, incorporation of lime upon desilication of concentrated aluminate solutions does not substantially improve desilication; neither it lowers losses of Na.sub.2 O, while losses of Al.sub.2 O.sub.3 are increased. A good effect is provided by lime only upon desilication of solutions with a concentration of at most 130-140 g/l of Al.sub.2 O.sub.3.
For solutions with 88.2 g/l of Al.sub.2 O.sub.3, 119.6 g/l of Na.sub.2 O and 3.86 g/l of SiO.sub.2, desilication at the temperature of 130.degree. C. with different additives provides the following results:
______________________________________ Additive of CaO, g/l 0 5 10 15 20 Silica ratio 140 185 188 385 413 Losses of Al.sub.2 O.sub.3, % 2.0 6.2 6.4 10.7 10.3 ______________________________________
Even in this case it is impossible to produce top-grade alumina. /cf. A. I. Liner "Alumina Production", Metallurgy Publishing House, Moscow, 1961, pp. 503-504/.
Also known is a process for desilication of aluminate solutions with the addition of white slime instead of lime.
Desilication of solutions with the addition of white slime is slightly improved. Losses of alkali and especially of Al.sub.2 O.sub.3 in desilication with the addition of white slime are substantially lower than in desilication with lime.
Though desilication with white slime is economically more efficient than that with lime, since lime is not consumed, it is still impossible to substantially purify solutions from silica. The silica modulus of the solution MSi is 500-600 /cf. A. I. Liner "Alumina Production", Metallurgy Publishing House, Moscow, 1961, p. 505).
A known process for desilication of aluminate solutions involves a two-stage treatment of aluminate solutions with calcium-containing additives. In the first stage of the process an aluminate solution is mixed with white slime resulting from the second stage of desilication and delivered to an autoclave desilication. The pulp from the autoclaves is fed to thickening and filtration. The filtered slime is repulped (liquefied) and employed in the production of alumina for the formulation of an alumina-containing charge. The filtered solution is subjected to desilication in stirred vessels under atmospheric pressure. As the desilication additive use is made of lime milk and a portion of the lime slime of the first desilication stage. A deeply-desilicated aluminate solution in the form of a pulp is delivered to thickening to a thickener unit. The thickened secondary lime slime is divided into two streams: one is fed to the first stage of desilication, the other to the second stage as a seeding agent. The aluminate solution effluent from the thickener units is subjected to a control filtration and delivered to carbonization (cf. I. Z. Pevzner, N. A. Makarov "Desilication of Aluminate Solutions", Metallurgy Publishing House, Moscow, 1974, L pp. 102-104).
However, a high-quality alumina cannot be produced on a commercial scale by the prior art process, since the final product is contaminated with silica. Furthermore, power and material consumption rates are also too high. Steam consumed per ton of Al.sub.2 O.sub.3 amounts to 1-1.5 Gcal; lime is consumed in an amount of 300-400 kg per ton of Al.sub.2 O.sub.3.